A method called an MS/MS analysis (or tandem analysis) is widely used as one of the mass spectrometric techniques for identification, structural analyses or quantitative determination of compounds having large molecular weights. There are various kinds of mass spectrometers with different configurations designed for the MS/MS analysis, among which tandem quadrupole mass spectrometers are characterized by their relatively simple structures as well as easy operation and handling.
As described in Patent Literature 1 or other references, in generally used tandem quadrupole mass spectrometers, ions generated from compounds in an ion source are introduced into a front-stage quadrupole mass filter (which is commonly represented as “Q1”), in which an ion having a specific mass-to-charge ratio m/z is selected as a precursor ion. This precursor ion is introduced into a collision cell containing an ion guide with four (or more) poles (this ion guide is commonly represented as “q2”). A collision-induced dissociation (CID) gas, such as argon, is supplied into this collision cell, and the precursor ion introduced into the collision cell collides with this CID gas, to be fragmented into various kinds of product ions. These product ions are introduced into a rear-stage quadrupole mass filter (which is commonly represented as “Q3”), whereby a product ion having a specific mass-to-charge ratio m/z is selectively allowed to pass through this filter, arrive at a detector and be detected.
An MRM measurement mode is one mode of the MS/MS measurement available in tandem quadrupole mass spectrometers. In the MRM measurement mode, the mass-to-charge ratio of an ion to be allowed to pass through is fixed in each of the front-stage and rear-stage quadrupole mass filters so as to measure the intensity (amount) of a specific kind of product ion produced from a specific kind of precursor ion. The two-stage mass filtering in the MRM measurement enables the removal of ions and neutral particles originating from impurities or compounds which are not the target of the measurement, so that ion intensity signals with high S/N ratios can be obtained. Due to this feature, the MRM measurement is particularly effective for the quantitative determination of a trace constituent.
Tandem quadrupole mass spectrometers can be used independently. However, they are often coupled with a liquid chromatograph (LC) or a gas chromatograph (GC). For example, the liquid chromatograph tandem mass spectrometer (LC/MS/MS), in which a tandem quadrupole mass spectrometer is used as a detector for a liquid chromatograph, is frequently used for a quantitative analysis of a compound contained in a sample which contains a large number of compounds or a sample which contains impurities.
When an MRM measurement is performed with an LC/MS/MS (or GC/MS/MS), a combination of the mass-to-charge ratio of a target precursor ion and that of a product ion must be set, as one item of the measurement conditions, in association with the retention time of an objective compound before the measurement of an objective sample is performed. By setting an optimum combination of the mass-to-charge ratios of the precursor ion and the product ion for each objective compound, the signal intensity of the ions originating from each objective compound can be obtained with high accuracy and high sensitivity, and the quantity of the compound can also be determined with high accuracy and high sensitivity. The combination of the mass-to-charge ratios of the precursor ion and the product ion can be manually set by analysis operators. However, the manual setting is considerably cumbersome and yet does not always ensure successful setting of an optimum combination. To address this problem, a system capable of an automatic and highly reliable setting of an optimum combination of the mass-to-charge ratios of the precursor ion and the product ion for an objective compound has been developed, as disclosed in Patent Literature 1.
A high-accurate and high-sensitive quantitative determination of an objective compound by an MRM measurement does not only require the aforementioned optimum setting of the combination of the mass-to-charge ratios of the precursor ion and the product ion for the objective compound but also the optimum setting of the collision energy and other measurement conditions for that combination. As described in Non Patent Literature 1, mass spectrometers having the function of automatically optimizing control parameters used in the MRM measurement have also been commonly known (this function is hereinafter called the “MRM measurement condition optimizing function”). In an example described in Non Patent Literature 1, the control parameters include the “Q1 pre-rod voltage” in the front-stage quadrupole mass filter, the “Q3 pre-rod voltage” in the rear-stage quadrupole mass filter and the “collision energy CE.”
Conventional MRM measurement condition optimizing functions can be classified into the following two methods:
(1) An analysis operator specifies a combination of the mass-to-charge ratios of the precursor ion and the product ion originating from a target compound. Then, an analysis of a known sample containing that compound (e.g. a standard sample) is performed so as to search for optimum values of the control parameters for the specified combination of the mass-to-charge ratios of the precursor ion and the product ion. The result is shown on a display unit.
(2) An analysis operator only specifies the mass-to-charge ratio of the precursor ion originating from the target compound. Then, a product-ion scan measurement of the specified precursor ion is performed using a known sample containing that compound to obtain a product-ion spectrum, and a predetermined number of product-ion peaks are selected from that spectrum in descending order of signal intensity. Subsequently, for each combination of the originally specified precursor ion and each of the selected product ions, the search for the optimum values of the control parameters is conducted and the result is shown on a display unit.
In the case of method (1), the analysis operator needs to have previous knowledge of not only the mass-to-charge ratio of the precursor ion originating from the compound but also that of the product ion. By contrast, method (2) has the advantage that an appropriate product ion is automatically searched for and appropriate values of the control parameters can be obtained even if the analysis operator does not know the mass-to-charge ratio of the product ion to be selected as a target. However, it should be noted that, in the case where a plurality of product ions are generated from one precursor ion, the product ion showing the highest signal intensity is not always the most suitable ion for quantitative determination; in some cases, a product ion with a lower signal intensity may have a higher degree of peak purity and be more suitable for quantitative determination. If the product ion most suitable for quantitative determination has such a low intensity that is not included in the predetermined number of ions selected in descending order of intensity, the optimum parameter values for that ion cannot be obtained by method (2).
In the previously described conventional MRM measurement condition optimizing function, it is assumed that the analysis is conducted in such a manner that a standard sample or a sample containing a single compound is introduced into the ion source of the tandem quadrupole mass spectrometer by infusion or flow injection. Therefore, the result of a series of measurements performed for one injection of the sample can be used to optimize the values of the control parameters for the MRM measurement for the mass-to-charge ratio of only one kind of precursor ion. If there are a number of precursor ions for which the values of the control parameters need to be optimized, the sample injection and the series of measurements need to be repeated as many times as that number, so that a considerable amount of time is required for optimizing the MRM measurement conditions.